Shade trees. The developer shall plant nursery-grown shade trees within the right-of-way on both sides of each street, eight feet in from the curbline, if possible, so as not to interfere with possible future utilities or sidewalks, not more than 50 feet apart. Shade trees to be of at least 2 1/2 inches in diameter, to be measured one-foot above grade, and of the following varieties: oak, linden, sycamore, maple (except silver maple) or locust, and shall be trimmed so lowest branches will be at least six feet above grade.